The invention relates to a piezoelectric motor particularly for use as a drive a motor for a vehicle or as a servo-motor and also for applications where a high drive power and/or high speeds as required. The piezoelectric drive can be a rotational drive or a linear drive.
A special and important field of application for the Invention resides in the use for example as electric motors for operating the brake discs in connection with hybrid motor vehicle drives.
A piezoelectric drive motor for high power applications which may be used for driving heavy loads is already known from WO 03/005 553 A2.
The piezoelectric drive described therein comprises a plurality of piezo packs in the form of stacks of piezo elements which are arranged between a relative stationary reference part and a drive part which is movable relative to the stationary part. The piezo packs provide for a selective generation of a clamping force between the reference part and the drive part and are therefore called clamping piezo packs. These clamping piezo packs are each connected with one end of the piezo element stack arrangement to the reference part and abut with the other end, the drive part without being firmly attached thereto. The orientation of the row in which the clamping piezo packs are arranged in side-by-side relationship is in a direction transverse to the relative movement between the drive part and the reference part.
Additional piezo packs which are also in the form of piezo element stacks and which are called stepping piezo packs because of their function are arranged with their stack height direction extending about normal to the stack height direction of the clamping piezo packs and are oriented in the direction of the relative movement between the reference part and the drive part. These stepping piezo packs are each connected with one end of their piezo element stack arrangement again at the reference part and connected with their other end to the end of a respective clamping piezo pack (or a respective sub-group of clamping packs) abutting the drive part.
There are two groups of clamping piezo packs and two groups of stepping piezo packs. They are electrically energized in such a way that in each case the one group of clamping piezo packs is energized, that is, expanded by the application of electric energy, whereby these clamping packs engage the drive part while the other group of clamping packs is de-energized that is in a release position in which they are not in clamping engagement with the drive part. Then the group of stepping piezo packs assigned to the clamping piezo pack which is energized is activated so that the respective stepping packs expand and, as a result, move the ends of the clamping packs engaged with the drive part by a certain distance in the direction of movement of the drive part. As a result, the drive part is moved by a corresponding distance relative to the clamping packs of the other clamping pack groups which are in their release position. Then the control is switched, that is, the previously energized clamping packs are de-energized that is switched to the release position, as well as the respective stepping piezo packs so that the clamping packs return to their original position while, at the same time, the other group of clamping packs is switched from its release position to its clamping position by energization thereof whereupon their stepping piezo packs are energized to activate them. In this way, the two groups of clamping piezo packs together with the associated stepping piezo packs provide, by their alternating energization, for a continuous series of steps for moving the drive part in a stepwise manner. Since the control of the piezo packs occurs in a frequency range of several kilohertz, for example, 25 kHz, a quasi-continuous movement of the drive part relative to the reference part is obtained.
In the piezo electric drive disclosed in WO 03/005,553 the reference part is connected to the stationary stack ends of the clamping piezo packets by way of a “semi-rigid” bridge plate. “Semi-rigid” means that the bridge plate adapts, at least in the length and expansion tolerance range of the clamping packs, in a static state to stack height differences of the clamping piezo packs by elastic bending, but, at the operating frequency, remains essentially rigid in the dynamic state. The reason herefor is that the movements of the piezo pack are very small. The available expansion length of a piezo pack is about one thousandth of the height of the pack. With a stack height of the clamping piezo pack of about 20 mm, an expansion length of about 20 micrometers is obtained. On the other hand, the tolerances of the mechanical components of the drive are in the range of hundreds of a millimeter even with highly precise manufacturing methods. Also, the stacking height of the piezo packs is subject to manufacturing tolerances so that, in a number of adjacently arranged piezo packs, they all have a different height within those tolerances. The arrangement according to WO 03/005 553 takes these facts into account by the provision of the semi-rigid bridge and in this way establishes the conditions for a practical functional capability of providing such a piezo electric drive which is suitable to generate a large drive power, because it provides for the necessary functional conditions that the clamping piezo packs between the reference part and the drive part are always maintained under high pretension since, with the available expansion stroke, the required clamping force and the friction force needed for the movement of the drive part could otherwise not be achieved. The drive part supporting the free ends of the clamping pack is stiff in relation to the semi-rigid bridge.
It is the object of the present invention to provide an improvement over the known principle of a piezo electric drive particularly with regard to an economical and practical manufacture of such piezo electric drives and, in view of the tendency of providing motor vehicles with hybrid drives. Hybrid drives can be selectively switched between an electric drive and an internal combustion engine. Piezo electric drives may also selected as electric drives in such hybrid concepts. Also the use of such electric drives in connection with vehicles equipped solely with internal combustion engines is being taken into consideration so that a further object of the invention resides in the provision of such a piezoelectric drive acting on a brake disc of a motor vehicle (or a similar drive disc).
An important point of the object presented above is that the arrangement of a piezoelectric motor is so selected that, depending on the application, small or large drive speeds or variable drive speeds can be generated. Also very high drive forces at low speeds can be generated.